Add ability to specify polling interval

Set with --interval for the standalone app or
RQ_POLL_INTERVAL in configuration specified with
RQ_DASHBOARD_SETTINGS.

Fixes #26
Merge pull request #28 from sburns/interval

Add ability to specify polling interval
Bump version to 0.3.1.
Refactor __init__ and init_app method
Type fix
Stick to the basics.